Intersections in road systems exist and are modeled in navigational and geographic databases. Intersections have different geometries and intersection characteristics. For example, some intersections are four way orthogonal intersections, in which two bi-directional roadways intersect at a right angle. In a four way orthogonal intersection vehicles or travelers may have the ability to take a right turn, a left turn, or a U-turn while traveling in any of the four total directions of the two roadways provide. In some intersections however, one or more of the turn possibilities are restricted and not allowed. Turn restrictions are in place for a multitude of reasons, many of which have little to do with the geographic characteristics of an intersection. For example, a turn restriction may exist because a planning or policy commission for a community has determined that an excessive amount of accidents are caused at an intersection. In such a case, a singular, or all, left turns may be restricted. Turn restrictions may also be in place because of traffic volumes at different times of day. For example, a left turn restriction may be put into place for the hours of 4:00 PM to 7:00 PM because there is a significant increase in traffic during this time of day, and a turn restriction is needed to allow for proper traffic flow.
The policy basis, or non-geographic reasoning, for putting into place turn restrictions makes it difficult to determine turn restrictions without manually determining the existence of a turn restriction at each intersection of a geographic or navigational database. Manually determining the existence of turn restrictions generally involves sending a person to an intersection to determine if a turn restriction exists, which is a resource intensive and expensive task.